问题
Is it possible that load a from region to reading pane when a custom button in a custom ribbon was clicked?
回答1:
No - you have to firmly associate a Form Region with one or more item types via the manifest settings for the Form Region. Although you can hide them on demand, you cannot load them on demand. If you want to demand-load your UI, use a custom Task Pane instead.
来源：https://stackoverflow.com/questions/24505586/display-form-region-with-a-button-from-ribbon-outlook-add-in